Comparative results between patients are typically assessed by the number of grafts transferred. Other times they involve the density of grafts in the recipient area. In fact, we know that individual hair characteristics are not the same.
It is the amount of hair grafts transferred that normally helps doctors assess the comparative results of various patients although there are times wherein the density of the hair grafts in the receiving area are also involved. It is a known that the characteristics of individual hair are different, as such, assessments made with traditional means can be called assumptions as well as both erroneous and unsound. Predictive values are of extreme importance since they are based on a more precise practice concerning individual hairs and their distinct natures. HMT or Hair Mass Transferred with every surgery is always taken note of and gives surgeons the means to assess the effectiveness of the surgery.
History
It was Dr. Bob Limmer that first took note of the average densities that resulted from the hair transplants of his patients. After the first session, his patients averaged 41 hairs per square centimeter. In the subsequent sessions, the hair density average rose to 50, 63 and then 81 hairs per square centimeter. What is most notable about the average densities taken by Dr. Limmer is that they are much lower than the usual densities in the typical donor or recipient area.
Manyy Marritt was another individual that studied illusions of coverage and hair removal. His studies concentrated on the coverage illusion through the plucking of hair in one square centimeter. What he discovered was that even after the plucking of over half of a person’s originally existing hair, the person still looked as if he had a full head of hair. It was not noticeable that any hair was plucked at all. It can be inferred from this study that the characteristics of each hair follicle are the same including hair diameter as well as follicular, calculated and hair density.
Coverage Prediction
The coverage provided by hair comes from the light waves being reflected off of the hair color that has a corresponding wavelength. When an individual notices the thinning his or her hair, it means that the light waves are being reflected not only by the color of the hair, but the skin or scalp as well. When light waves are reflected by only the color of the skin or scalp, it is called baldness. Doctors can replicate an illusion of coverage by moving enough hair follicles to the areas experiencing balding or thinning. Once enough hair is there to reflect the light waves, the illusion of coverage is achieved.
Math can be applied to the illusion of coverage to understand it better where in each strand of hair is understood to be a cylinder so that its volume and surface area can be computed. To compute for volume and at the same time compute for the mass of each strand of hair, the following equation is used:
Volume – pr^{2}h
The diameter is also influential to the volume of hair because even the most minute changes in the diameter of a strand of hair. If the diameter is doubled, the volume of the hair strands quadruple. In the same way, if due to androgenic alopecia the diameter of the hair decreased by half, the hair will look like it only has a quarter of its original volume. With a thorough understanding of how hair diameter dictates the level of coverage, doctors can start simulating coverage when a high enough mass of hair is transferred to the thinning or balding areas of the patient’s head.
Doctors can prevent light waves from reflecting off of the patient’s scalp when the hair follicles are placed on the patient’s head in neat rows rather than stacked on top of one another. Take, for example, hair when it is wet. The moment hair is soaked in water, it has a tendency to start clumping or stacking. It is the clumping of the hair that makes it look so thin when it is wet. To determine the hair surface area of an individual, the following equation is applied:
Hair Surface Area = 2 p r + 2 p r h
In the above equation, r pertains to the radius of the hair strand while h applies to the length of the strand. To compute for the amount of hair transfers needed to cover a bald area, the physician will need the following information from the patient:
 Average hair diameter
 Bald surface area
 Total number of hairs transferred
 Length of hair
When the doctor is able to compute for the surface area of the head that is bald, it then becomes possible for them to make a precise estimate of the hair transfers that will be needed to give a more complete coverage. Hair length for the transfers can be varied since the reflective surface area in any length of hair can be predicted.
We are quite lucky that our creator used cylinders to cover our spherical scalp. Archimedes pointed out that the sphere is "inscribed" in the cylinder. It's north pole just touches the top of the cylinder; the south pole just touches the bottom. And the cylinder and sphere just barely make contact all along the equator. If the cylinder were the least bit shorter or skinnier, the sphere would not fit inside. If r = the radius of the sphere and the radius of the cylinder, then 2r = the height of the cylinder, V_{c} = volume of the cylinder =(¼r^{2})(height) = 2¼r^{3}, and V_{s} = volume of the sphere = (4/3)¼r^{3}. Therefore, V_{c}/V_{s} = 3/2, the ratio Archimedes was most proud of. The can will hold 50% more soda pop than the ball. It is interesting to note that the ratio of the cylinder's area to the sphere's area is the same as the ratio of their volumes: A_{c} = area of top + area of bottom + area of side = ¼r^{2} + ¼r^{2} + (2¼r)(2r) = 6¼r^{2} and A_{s} = 4¼r^{2}, so A_{c}/A_{s} = 3/2. It takes 50% more paint to decorate the can than to decorate the ball. It follows that it requires 50% less cylindrical hair to cover the spherical scalp.Interestingly, these follicular groups are arranged in mathematical spirals, another complex calculation developed by Archimedes, but this is beyond the scope of our discussion.
The term mass is actually a measure of volume. A mass of 1 gm is a cubic centimeter of water. It follows that hair mass and hair volume are essentially the same. In 1998, Dr. John Cole introduced Hair Mass Transferred and Total Hair Volume Transferred as a predictor of hair transplant coverage. The problem with the Cole method was it relied on the mean hair diameter, a variable that required tedious measurements with potentially costly equipment. In May 2001 Dr. James Arnold presented Hair Mass Index (HMI) as an ingenious means to quickly and inexpensively asses Hair Mass. He measured HMI in both the donor area and recipient areas of his patients. The Arnold method may not be as precise as the Cole method and does not evaluate the actual mass of hair that is transferred at the time of surgery. Rather, it assesses the individual’s hair mass before surgery and after regrowth of the transplanted grafts. Interestingly, he noted a lower than expected HMI in the recipient area than was predicted based on the number of grafts and hairs he transferred in many of his former patients. This was the forefather to objective efficiency evaluations. Dr. Frank Neidel wrote a chapter about HMI in the most recent edition of Hair Transplantation of Dr. Walter Unger and Dr. Ron Shapiro. HMI measures the hair mass in the donor area and recipient area, but it does not measure what is actually transferred.
Cole has stated that scalp hairs may be classified according to the following table:
Very fine

Fine

MediumFine

Medium

MediumCoarse

Coarse

<60um

6065um

6570um

7075um

7580um

80um>

It should be noted that hair on other parts of the body has a much different diameter than scalp hair. This characteristic offers great potential coverage from coarse hair sources such as from the chest.
Neidel notes that HMI can be classified according to table below:
Optical effect of fine hair 
Optical effect of normal hair 
Optical effect of thick hair 
0.180.32 
0.320.5 
0.50.72 
Methods:
Measurement of mean hair diameter requires a sample size of 20 hairs. We have found in multiple evaluations that this sample size results in a more precise average hair diameter. The hair is measured with the help a micrometer. The measurements are very precise with an error of about 1 micron. Mean hair diameter is equal to the sum of 20 hair diameters divided by 20. Hair diameters are measured in micrometers. Cole recognizes the extreme variability of hair diameters. For this reason, he believes that when one measures only 20 hairs, you should not include hair diameters that are particularly small or extremely large so that you get a more accurate estimate of the average diameter.
This measurement allows us to calculate the mean surface area and mean hair volume for any patient provided the length of hair is known. Hair length varies from one individual to another. Therefore, we introduce the term hair mass transfer index (HMTI) to compensate for hair length. HMTI assumes a standard length of 1 cm. It is easy to compensate for any length of hair as we will show. While cutting the grafts, assistants examine and write down the number of hairs contained in each graft. The grafts may be individual follicular units, double follicular units (DFU), multiple follicular units, (MUG). In addition, you may include fractionated follicular units that may include a variety of combinations to include three hair follicular units fractionated into three one hair grafts. The objective is to assess the total number of hairs that are transferred of a particular graft type or size. On a statistical point of view, it is worth using this method for the first 200 grafts of any particular type (FU, MUG, etc.) and then it is possible to proceed to an extrapolation. Single hair grafts that are obtained by fractionation should always be counted in their entirety. This sum is the Total Hairs Transferred (THT). Mean hair radius is the quotient of the mean hair diameter divided by 2.
Mean hair volume index (MHVI) is the product of the square of the mean hair radius, p, and a hair length of 1 cm.
MHVI = ( r 2 ) ( p ) ( 1 )
Hair mass transferred index is the total hairs transferred (THT) and mean hair volume index (MHVI).
HMTI = (THT) ( MHVI)
The expected actual hair mass you have transferred (HMT) may be easily calculated based on any hair length. One simply multiplies the HMTI by the actual length of hair on the patient. Now one has the capacity to evaluate efficiency of their hair transplant. One can calculate the hair mass index in the recipient area and compare it to the actual HMT calculated at the time of surgery. This method allows one to evaluate efficiency on multiple regions of the scalp.
These are two old and two recent examples of predicted Hair Mass Transferred: